Method for compressing wooden elements

ABSTRACT

A method of compressing at least one flat wooden element having knots therein to permanently increase the hardness of the wooden element. The method includes the steps of inserting an elastic material layer between a pair of spaced apart press plates which are moveable with respect to each other, the elastic material layer being softer than the wooden element. The wooden element is then interposed between the elastic material and one of the press plates, and the pair of press plates displaced toward each to a predetermined spacing thereby compressing the entire surface of the wooden element and elastic material layer, the thickness of the elastic material layer in the compressed state being at least one-half the thickness of the compressed wooden element. Portions of the knots in the wooden element are forced out of the element and into the elastic material layer without being broken to shivers by the pressure exerted thereon. The portions of the knots projecting from the surface of the wooden element and received in the pad during the displacement step are then removed.

FIELD OF THE INVENTION

This invention relates to a method for compressing board-shaped orotherwise flat wooden elements in order to produce a remainingcompression of the wood and thereby permanently increase the hardnessthereof. In particular the invention is applicable to comparatively softwood such as pinewood and whitewood.

DESCRIPTION OF THE PRIOR ART

German patent specification No. 601162 and U.S. Pat. Nos. 1,403,722 and2,666,463 disclose that it has been previously known to compress woodenelements to increase the hardness and the mechanical strength thereof.This is done quite simply by introducing the elements in question one byone between two mutually moveable press plates and subjecting them to anappropriate pressure.

The prior art technique such as represented not only by theabove-mentioned patent specifications but also by Swedish patentapplication No. 7805483-0 is however limited to the use of uniformknotless wood; a material which on one hand fetches a high price and onthe other is not desirable on the whole within many fields ofapplication, since knots usually lend a characteristic, vivid andaesthetically attractive appearance to the wood. This limitation to theexpensive, aesthetically not very striking wood qualities that areknotless has led to the fact that the technique in question has beencarried into effect to an extremely small extent in practice.

Swedish patent specification No. 104506 discloses a method ofcompressing wooden elements at high pressures between reciprocatingpress plates. These elements are however not board-shaped or flat butconsist of irregularly split logs which are charged to the press deviceat random without any specific knot-receiving layer between each elementand the adjacent press plate. The logs treated in accordance with thismethod are moreover intended for the production of pulp, while the boardelements treated in accordance with the present invention are intendedto be used e.g. in floors, as furniture components, etc.

German patent specification No. 1075821 discloses how a plurality offlat wooden elements or boards are pressed together between tworeciprocating press plates. The pressure used in this process is,however, extremely mediocre and not at all sufficient to produce apermanent compression completely throughout the wood material whilereducing the thickness thereof by 30 to 50% or even to bring about theslightest tendency of any knots to come out of the surface of the board.

U.S. Pat. No. 2,974,697 and German Offenlegungsschrift No. 2112618disclose how wooden elements are treated by being fed between rollers.None of these publications deals with the problem of keeping the knotsof the wood intact during pressing. Quite to the contrary theOffenlegungsschrift No. 2112618 aims at eliminating the knots bycrushing them during the pressing between the rollers.

SUMMARY OF THE INVENTION

An object of the present invention is to make it possible to compresseven wood which is full of knots and thereby provide wooden elementshaving good impact and scratch resistance properties. According to theinvention this is achieved by means of a method which is characterizedby the steps of subjecting the individual wooden element to a pressurein the region of 20 to 60 MPa (Mega Pascal) during pressing between twopress plates which are moveable relative to each other in a manner knownper se and keeping it therebetween together with at least one layer ofmaterial, the hardness of which is less than the hardness of the knotsexisting in the element in question and which layer is thick enough toallow portions of the knots to be forced out of said element and intosaid layer without breaking to shivers by the pressure exerted.

By using a pressure in the region of 20 to 60 MPa, preferably 30 to 40MPa, a substantially homogeneous compression throughout the woodmaterial is achieved, leading to a permanent decrease in the thicknessof ordinary wood amounting to 30 to 50%. At the same time the knotsremain perfectly undestroyed due to the fact that they are notcompressed directly between the two press plates of hard material, butare accomodated in said receiving layer consisting of a material whichis softer than the knots themselves. The portions of the knots that willthereby project from a surface of the wooden element thus treated maylater on either be removed, e.g. by grinding, or quite simply be left inunchanged condition so as to be subsequently pressed into suitablesupports consisting of a comparatively soft material, e.g. in connectionwith fixing the wooden element to the support by gluing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partially cut perspective view of a preferred embodiment ofthe press device used for carrying out the method of the invention;

FIG. 2 is a cross-section along the lines II--II of FIG. 1 showing aboard to be treated;

FIG. 3 is a similar cross-section illustrating the same board during thepressing treatment;

FIG. 4 is an analogous cross-section showing an alternative embodimentof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The press device 1 shown in FIGS. 1 to 3 comprises in the conventionalmanner two press members of hard material, such as steel, which aremoveable in relation to each other. More exactly said members consist ofa stationary plate or table 2 and a press plate 3 which is verticallymoveable to and fro in relation to the table. The plate can be subjectedto a press force of suitable magnitude in any arbitrary manner (notillustrated). On one of the press members, in this case on the table 2,two limiting members 4,5 in the form of long borders or bars areprovided, said members being fixed to the table in any arbitrary manner,e.g. by means of screws, welds or the like. At their upper and inneredges the borders 4,5 present chamferred surfaces 6 the purpose of whichis to facilitate the introduction of a board between the borders. FIG. 1illustrates the fact that no end limiting means are provided between theborders 4,5 meaning that between said borders a board receiving space isconfined which is open at the opposite ends thereof.

FIG. 1 further shows that a pad 7 is located between the two sidelimiting borders, said pad being intended to form a bed for the board tobe received. In accordance with the invention this pad consists ofplastic or rubber, preferably neoprene; that is, a material the hardnessof which is less than the hardness of the knots included in the boardsto be pressed. In this connection the definition "hardness" should beconstrued not only as the Brinell-hardness but also as the modulus ofelasticity of the materials in question, i.e. both of these magnitudesof the material of the pad 7 should be positively smaller than thecorresponding magnitudes of the knots. Another important criterion ofthe pad 7 is that it should have a sufficient thickness in relation toits own elasticity characteristics and the pressure exerted to be ableto accomodate knot portions of considerable sizes even when the paditself is compressed.

Now reference is made to FIGS. 2 and 3 which illustrate the varioussteps during a pressing operation. FIG. 2 shows how the press device ischarged by introducing a board 8 between the table 2 and the press plate3 and letting the board rest on the elastic pad 7. This introduction isfacilitated by the chamferings 6. Thereafter, the plate 3 is subjectedto a press force which, after having brought said plate to the initialposition shown in FIG. 2, initiates a pressing work during which theboard 8 as well as the pad 7 are subjected to a pressure of suitablemagnitude and duration, said work being terminated in the phaseillustrated in FIG. 3. In the next phase the pressure is reduced and theplate 3 is caused to return to the rest position of FIG. 2 in which thepressing cycle or operation is terminated.

According to tests performed the maximum pressure that may occur duringthe above-mentioned cycle should not greatly exceed 60 MPa (8700 psi).On the other hand the pressure should not be less than 20 MPa (2900psi). Preferably the active pressure is within the region of 30 to 40MPa in connection with the compression of such comparatively soft andporous sorts of wood as pinewood and whitewood (dry weights by unit ofvolume approximately 0.49 and 0.43 kg/dm³ respectively). In connectionwith harder sorts of wood higher pressures may be used, while softersorts of wood may require lower pressures. Tests have further shown thatthe pressing without disadvantage may be carried out instantaneously orsubstantially instantaneously, i.e. the press plate 3 may be moved witha suitable speed without interrruptions from the initial position shownin FIG. 2, turned and removed back to the point where the plate leavesthe board 8.

Under the above-mentioned conditions it is possible to achieve apermanent compression of the treated board amounting to 30 to 50%, i.e.a board the thickness of which is e.g. 10 mm will be compressed to athickness of 7 to 5 mm. The pressing method described is particularlyapplicable to so called "thin-boards", i.e. flat wooden elements whichare relatively wide and long and on one hand are positively thinner thanconventional sawngoods (usually 20 mm or more) and on the other thickerthan veneer of the usual type (1 to 3 mm). Thus the sizes of thestarting material, that is the boards to be introduced into the pressdevice of FIG. 2, may advantageously amount to 5 to 15 mm in thicknesswhile the width amounts to minimum 120 mm. In this connection it shouldbe noted that the board shown in the drawing for the sake of savingspace in the drawing has been given an exaggerated thickness in relationto the width.

The process of squeezing knots out of the proper board 8 taking place inaccordance with the invention is illustrated in the drawing by thesingle knot 9, though it should be understood that each board mayinclude a great number of such knots.

The material of the pad 7 which, by its relative softness or elasticity,is able to accomodate the projecting portion of the knot 9 shouldpreferably be homogeneous or structurally uniform. In practice the padmay consist of neoprene though other suitable elastic material from thegroup consisting of rubber or plastics may also be used. The materialshould be selected so that the pad 7 in its compressed condition (seeFIG. 3) will have a thickness amounting to at least 50% of the thicknessof the board in its compressed condition. Otherwise there is a risk thatthe knots will be split. From FIG. 3, it further appears that the totalthickness of the pad 7 and the board 8 when compressed to a maximum isat least slightly greater than the height or thickness of the limitingborders 4,5 so that these cannot be used safely as spacing means betweenthe press members 2 and 3.

Due to its elastic nature the pad 7 will to a certain extent act as aspring that presses up or tends to press the board up out of the spacebetween the borders when the pressing operation is finished.

In the press device shown, boards having a predetermined standard widthare compressed, either one single long board or a plurality of shorterboards being treated on each occasion. If it is desirable to adapt thedevice to varying board widths it is of course possible to mount atleast one of the borders detachably on the table so as to be adjusted toanother width.

FIG. 4 illustrates an alternative embodiment of the invention in which apad 7' is connected to the moveable press plate 3 instead of beingattached to the stationary table 2. An advantage of this embodiment isthat the projecting knot portions squeezed out will be directed upwardlyfrom the lying board. In a line production this will facilitate theremoval of said knot portions, e.g. by grinding, since then the boarddoes not have to be turned before the grinding operation.

The boards or wooden elements treated in accordance with the presentinvention are well suited to be used as surface forming members infloors, said floors attaining excellent properties regarding impact andscratch resistance in comparison with floors made from conventionalsoftwood, such as pinewood or whitewood, while at the same time fullymaintaining the attractive appearance of wood which may be rich inknots. They may also be used as surface forming members in tables or anyarbitrary furniture objects needing good-looking and resistent surfaces.In all applications the boards compressed may be mounted together on acommon support consisting of a cheaper material, such as fibre board,thereby providing economical production.

It is possible to treat during a single pressing operation in one andthe same press device two or more wooden elements arranged in layers oneabove the other, thereby increasing the capacity of the device. In thiscase one introduces between the two press members a packet built up froma first pad, a first wooden element, a first hard insert plate (e.g. asteel plate), a second pad, a second wooden element, a second hardinsert plate, etc, to the height desired.

I claim:
 1. The method of compressing at least one flat wooden elementhaving knots therein comprising the steps of:inserting an elasticmaterial layer between a pair of spaced-apart press plates which aremoveable with respect to each other, said elastic material layer beingsofter than said wooden element; interposing said wooden element betweensaid elastic material layer and one of said press plates; displacingsaid pair of press plates toward each to a predetermined spacing therebycompressing the entire surface of said wooden element and elasticmaterial layer, the thickness of said elastic material layer in thecompressed state being at least one-half the thickness of the compressedwooden element, portions of the knots in said wooden element beingforced out of said element and into said elastic material layer withoutbreaking to shivers by the pressure exerted thereon; and removing theportions of said knots projecting from the surface of said woodenelement and received in said pad during said displacing step.
 2. Themethod according to claim 1 wherein a plurality of wooden elements aresimultaneously compressed between said press plates, said press platesbeing composed of metal; and wherein an elastic material layer isprovided between each of said plurality of wooden elements and a pressplate.
 3. The method according to claim 1 wherein said elastic materiallayer comprises a pad composed of a material selected from the groupconsisting essentially of plastics and rubber.
 4. The method accordingto claim 3 wherein said elastic material is composed of neoprene.